1. Field of the Invention
The invention is in the field of heat transmission and transport using loop heat pipes.
2. Description of the Prior Art
The loop heat pipe (LHP) is a thermal control and heat transport device initially developed in Russia. Its original purpose was to provide passive (no moving parts) cooling for a missile. It was later used by the Russians for spacecraft cooling. It has since been fabricated and tested by companies in the U. S. It has been space flight tested in Space Shuttle Hitchhiker Canisters and will be used in a number of spacecraft missions. The LHP can transport large quantities of heat over long distances with moderate temperature difference, and can be designed to be mechanically flexible.
FIG. 1 shows a schematic of a typical LHP. It consists of an evaporator with a porous wick, a contiguous compensation chamber, condenser, and vapor and liquid transport lines. A two-phase (liquid and vapor) working fluid, such as ammonia, is used. Heat applied at the evaporator wall causes vaporization of the liquid at the outer surface of the wick. This vaporization and fluid surface tension causes a curved meniscus to form in the wick. The pressure rise due to this curved meniscus drives fluid to circulate about the loop. The smaller the pore size of the wick, the greater the pressure rise that can be generated. Heat removal causes the liquid to condense, and sets up a steady fluid motion.
FIG. 2 is a scanned image of a photograph of the evaporator-compensation chamber assembly (including heater plate) of a Russian LHP. The compensation chamber is a separate element with a larger diameter than the evaporator. FIG. 3 shows a possible adverse vapor-liquid configuration in this assembly in the micro-gravity (near 0-g) condition of space. This configuration is adverse in that the liquid in the compensation chamber is separate from and does not wet the evaporator wick. Of course, other vapor-liquid configurations in micro-gravity, many of which wet the wick, are possible. However, spacecraft components must always be designed to operate under the worst possible condition. Similarly, FIG. 4 shows an adverse vapor-liquid configuration in 1-g (earth gravity); this is caused by the orientation (tilt) of the assembly with respect to the earth's gravity vector. Other orientations in earth gravity, as shown for example by the horizontal orientation of FIG. 5, can result in an acceptable vapor-liquid location. Because of evaporator non-wetting illustrated by FIG. 4, LHP usage in 1-g conditions has been constrained to orientations that are near horizontal or where the compensation chamber is above the evaporator.
The above noted deficiencies of LHPs have prompted both Russian and U. S. researchers to seek corrective measures. These have usually consisted of the incorporation of an auxiliary or secondary wick. The principal behind this secondary wick is illustrated by FIG. 6. This shows liquid flowing under capillary pressure from a larger to a smaller pore. The pressure drop going from vapor to liquid in the large and small pores is given by .DELTA.P.sub.1 =2.sigma. cos .theta..sub.1 /R and .DELTA.P.sub.2 =2.sigma. cos .theta..sub.2 /r, respectively. Here .sigma. is the surface tension, .theta. the contact angle, and R and r are the radii of curvature, respectively. With the vapor pressure the same in the two pores, .DELTA.P.sub.1 =P.sub.v -P.sub.L1 and .DELTA.P.sub.2 =P.sub.v -P.sub.L2. Equating P.sub.v in the two equations for the same contact angle, .theta., in the two pores, there results P.sub.L1 -P.sub.L2 =2.sigma. cos .theta.(1/r-1/R). Pressure within the liquid is higher in the large pore than in the small one and hence liquid flow ensues in that direction.
The Russian version of this wick follows from their powder metal technology. FIG. 7 shows two such wicks, one for each compensation chamber in a dual compensation chamber LHP. The wicks, shown by the coarse crosshatching, occupy the annular region of each compensation chamber, butting against the main or primary wick in the evaporator. Properties of these wicks are: 93% porosity, 600 microns effective pore diameter, and 1.5.times.10.sup.-5 meter.sup.2 permeability. For comparison the corresponding properties of the primary wick, the driving capillary force in the LHP, are: 72% porosity, 2.3 microns effective pore diameter, and 4.times.10.sup.-14 meter.sup.2 permeability.
The secondary wick of FIG. 7, by containing liquid within its pores, does provide interface control within the compensation chamber. However, as regards the liquid supply to the evaporator, its properties are a compromise between micro-gravity and 1-g requirements, and thus do justice to neither. Moreover, the design is deficient in that the secondary wick merely butts, but does not overlap, the primary evaporator wick.
In micro-gravity, capillary driven flow must overcome only the pressure loss in the medium through which it is flowing, i.e., there is no hydrostatic (gravity) head loss. The capillary pressure difference driving the flow is given for liquids that wet perfectly by .DELTA.P=4.sigma./d, while the laminar flow pressure loss is given by .DELTA.P=.mu.uL/K. Here, .sigma. is the surface tension, d is the pore diameter, .mu. is the liquid viscosity, u is the liquid velocity, L is the length traversed, and K is the permeability. The permeability is inversely proportional to the flow resistance of the medium and is given by K=.epsilon.d.sub.h.sup.2 /32, where .epsilon. is the porosity and d.sub.h the hydraulic diameter of the medium. For randomly packed spheres permeability is given approximately by K=0.00667d.sup.2.epsilon..sup.3 /(1-.epsilon.).sup.2. Solving for the resultant velocity in the medium, it is found that u=(4)(0.00667).sigma.d.epsilon..sup.3 /.mu.L(1-.epsilon.).sup.2. Thus it is seen that velocity increases as pore diameter, d, increases.
In 1-g, capillary driven flow must overcome both flow pressure loss and hydrostatic head due to gravity. Velocity is now given by u=[0.00667.sigma.d.sup.2.epsilon..sup.3 /.mu.L(1-.epsilon.).sup.2 ][4.sigma./d-.DELTA..rho.gL], where .DELTA..rho. is the difference between liquid and vapor density, and g is the acceleration due to earth gravity, 9.8 meter/second.sup.2. The dependence of liquid velocity on pore diameter is now more complex. Indeed, unless the pore diameter is sufficiently small such that 4.sigma./d is greater than .DELTA..rho.gL there is no flow. Where the hydrostatic term, .DELTA..rho.gL, becomes significant, pore diameter must be small rather than large to cause liquid to flow. This is just the opposite of the result found for the micro-gravity case. Thus, the design approach taken entails the choice of secondary wick pore size that is a compromise between two conflicting requirements.
With an analysis similar to that above for effective pore diameter, it can be shown that it is much preferred that the secondary wick overlap the primary wick, rather than butting it. It was seen above that the permeability of the secondary wick can be orders of magnitude greater than that of the primary wick (1.5.times.10.sup.-5 versus 4.times.10.sup.-14 meter.sup.2). With overlap, the supply liquid within the secondary wick encounters much less flow resistance in reaching the far end of the primary wick than if it had to traverse the much denser primary wick. The overlapping wick does, however, suffer from the pore diameter compromise discussed above.
The U.S. approach to secondary wick design is closely held and rarely revealed. However, the designs appear to use 100 to 200 mesh screens rolled or formed to create channels or arteries. They appear to extend from the compensation chamber along most of the length of the primary wick, making only partial or sector contact.
Designs of this type cannot have much of a static wicking height capability, as pore size is determined by the gap between the screen layers. At best, this gap can be taken to be of the order of the wire diameter, 114 microns for a 100-mesh screen. The resultant static wicking height in ammonia at 25.degree. C. is 2.6 cm.
These designs are then primarily for micro-gravity or for near horizontal orientations of the compensation chamber-evaporator assembly in 1-g. They are of little or no utility for compensation chamber-evaporator orientations where the compensation chamber is below the evaporator. Additionally, contact between the secondary and primary wicks within the evaporator appears to be irregular, sector contact.
An alternate approach for liquid supply to the evaporator wick for any orientation of the compensation chamber-evaporator assembly in 1-g is the use of dual compensation chambers. Such an assembly was shown in FIG. 7. (Yuri Maidanik et al, Institute of Thermal Physics, Ural Division of Russian Academy of Sciences, Technical Report for Stage 2 of Project No. 473 for the International Science and Technology Center, Moscow, Russia, 1997). A photograph of the entire LHP with this assembly is shown in FIG. 8. The premise behind this design is that, for orientations of the assembly away from the horizontal, one of the two compensation chambers is always above the evaporator. Possible orientations of a dual compensation chamber LHP are shown in FIG. 9.
The obvious penalty of a dual compensation chamber LHP is the weight of the second compensation chamber and the liquid contained therein. Recent performance tests at the Air Force Research Laboratory have revealed an additional, significant penalty. This is shown, for example, for a -40.degree. C. condenser temperature in FIG. 10, where steady-state saturation temperature is plotted against power for the nine orientations of FIG. 9. Saturation temperature is seen to vary widely. For orientations 5, 6 and 8--whose common feature is a vertical evaporator with liquid return from below--this temperature is always hotter than the ambient (18 to 23.degree. C.). For orientations 3, 4, and 7--whose common feature is condenser above evaporator--this temperature can be quite cold, approaching -30.degree. C. at low power. For a number of applications such wide temperature variation is a serious problem or is entirely unacceptable.